1. Field of the Invention
The present invention relates to a cellular communication system, and more particularly, to an apparatus and a method for controlling transmission power in an Orthogonal Frequency Division Multiple Access (OFDMA) cellular communication system using a multiple frequency reuse factor.
2. Description of the Related Art
Recent developments in wireless networks focus on a 4th Generation (4G) communication systems that provides superior Quality of Service (QoS) at a higher transmission rate. In particular, the focus is on providing subscribers with high speed services by ensuring mobility and QoS to wireless Local Area Network (LAN) communication systems and wireless Metropolitan Area Network (MAN) communication systems, which network topologies that can provide services at a relatively high transmission rate.
To support broadband transmission networks for a physical channel of the wireless MAN communication system, the Institute of Electrical and Electronics Engineers (IEEE) has suggested using an Orthogonal Frequency Division Multiplexing (OFDM) scheme and an OFDMA scheme with an IEEE 802.16a communication system. According to the IEEE 802.16a communication system, the OFDM/OFDMA schemes are applied to the wireless MAN system to transmit a physical channel signal at a high transmission rate by using a plurality of sub-carriers.
The IEEE 802.16a communication system is based on a single cell structure without taking mobility of a Subscriber Station (SS) into consideration. In contrast, an IEEE 802.16e communication system does take mobility of the SS into consideration while still incorporating the features of the IEEE 802.16a communication system.
The IEEE 802.16e communication system reflects the mobility of the SS under a multi-cell environment. To provide mobility to the SS under the multi-cell environment, the operational relationship between the SS and a Base Station (BS) must be changed. To that end, research focuses on SS handover. A mobile SS is referred to as a Mobile Subscriber Station (MSS).
Hereinafter, a conventional IEEE 802.16e communication system will be described with reference to FIG. 1.
FIG. 1 is a schematic view illustrating the structure of a conventional IEEE 802.16e communication system.
Referring to FIG. 1, the conventional IEEE 802.16e communication system has a multi-cell structure including a cell 100 and a cell 150. The conventional IEEE 802.16e communication system includes a BS 110 for managing the cell 100, a BS 140 for managing the cell 150, and a plurality of MSSs 111, 113, 130, 151 and 153. The BSs 110 and 140 communicate with the MSSs 111, 113, 130, 151 and 153 through the OFDM/OFDMA schemes.
The conventional IEEE 802.16e communication system performs an Inverse Fast Fourier Transform (IFFT) and uses 1702 sub-carriers. Among the 1702 sub-carriers, 166 sub-carriers are used as pilot sub-carriers and 1536 sub-carriers are used as data sub-carriers. In addition, the 1536 sub-carriers are divided into 32 sub-channels, each including 48 sub-carriers. The sub-channels are allocated to the MSSs according to the state of the system. Herein, the sub-channel is a channel with at least one sub-carrier. For instance, 48 sub-carriers may form one sub-channel.
As mentioned above, when the sub-channels are formed in the IEEE 802.16e communication system, the total sub-channels are divided into several groups and mutually different frequency reuse factors are applied to each group. Hereinafter, a method of allocating frequency resources based on multiple frequency reuse factors in the conventional IEEE 802.16e communication system will be described with reference to FIG. 2.
FIG. 2 is a schematic view illustrating a frequency resource allocation operation based on multiple frequency reuse factors in the conventional IEEE 802.16e cellular communication system.
Referring to FIG. 2, a cell center region 200 adjacent to a BS has a relatively high signal to interference and noise ratio (SINR), so a frequency resource with a frequency reuse factor of 1 is allocated to an MSS located in the cell center region 200. In contrast, a cell boundary region 250, which is relatively remote from the BS, has a relatively low SINR, so a frequency resource with a frequency reuse factor greater than 1 (K>1) is allocated to the MSS located in the cell boundary region 250. By allocating the frequency resources to the MSSs with mutually different frequency reuse factors, limited frequency resources are used more effectively and efficiently.
Hereinafter, a method for creating the sub-channels based on the multiple frequency reuse factors in the conventional IEEE 802.16e communication system will be described with reference to FIG. 3.
FIG. 3 is a schematic view illustrating a procedure of creating the sub-channels based on multiple frequency reuse factors in the conventional IEEE 802.16e communication system.
Referring to FIG. 3, if the IEEE 802.16e communication system uses N sub-carriers, the N sub-carriers are divided into G groups. Each of the G groups consists of S sub-carriers, so that the following equation is satisfied.N=S×G.
A first sub-channel is created by selecting one sub-carrier from each of the G groups. A second sub-channel is created by selecting one sub-carrier from each of the G groups, except for the sub-carrier allocated to the first sub-channel. The above procedure may be repeated until all sub-carriers of the G groups are allocated to the sub-channels. As a result, a set of S sub-channels is created.
It is also possible to create a new set of S sub-channels having sub-carriers different from the above sub-carriers by varying the sub-carrier selection scheme. The number of sets of the S sub-channels including mutually different sub-carriers is (S!)G. Herein, a combination of the sub-carriers forming the sub-channel will be referred to as a “sub-carrier combination”.
In the following description, a set of nth sub-channels selected from among the (S!)G sets of the S sub-channels is defined as An and an mth sub-channel of the sub-channel set An is defined as SCmn. Herein, n=[0, (S!)G], and m=[0, S−1]. S sub-channels (SCmn and SCln) forming the same sub-channel set An are orthogonal to each other. So, the sub-carriers forming each of the S sub-channels may not collide with each other. In addition, the sub-channels (SCmn and SClk, n≠k) forming mutually different sets of the sub-channels are aligned without ensuring orthogonality therebetween; the sub-carriers forming mutually different sub-channels may collide with each other.
In addition, C sub-channel sets An are selected from among the (S!)G sets of the S sub-channels. At this time, if a predetermined sub-channel is selected from each of the C sub-channel sets An, the number of sub-carriers having the collision characteristics can be uniform. As a result, the total number of sub-carriers with collision characteristics between two sub-channel sets is proportional to the number of sub-channels; the sub-channel set is created by selecting sub-carriers from among the (S!)G sets of the S sub-channels. The C sub-channel sets with mutually different sub-carrier combinations, and the sub-carriers with uniform collision characteristics can be created through various schemes.
Hereinafter, a method of managing the sub-channel with a frequency reuse factor of 1 in the IEEE 802.16e communication system will be described.
First, when the frequency reuse factor is 1, all sub-carriers in a predetermined cell of the IEEE 802.16e communication system (that is, all sub-channels) can be used in adjacent cells. If each the cells uses a sub-channel set having the same sub-carrier combination (that is, if each cell uses the same An), interference variation may occur in each sub-channel of the sub-channel set depending on the channel states. Therefore, when presently measured channel information is applied to a next time duration, it is impossible to predict the channel state.
Hereinafter, a method of creating the sub-channel when the frequency reuse factor is 1 in the IEEE 802.16e communication system will be described with reference to FIGS. 4A and 4B.
FIG. 4A is a schematic view illustrating a procedure of creating the sub-channel when the frequency reuse factor is 1 in the conventional IEEE 802.16e communication system.
Referring to FIG. 4A, if the IEEE 802.16e communication system uses N sub-carriers, C sub-channel sets An can be created from the N sub-carriers through various sub-carrier selection schemes.
FIG. 4B is a schematic view illustrating a sub-channel set corresponding to FIG. 4A allocated to cells forming the IEEE 802.16e communication system.
Referring to FIG. 4B, the C sub-channel sets An are allocated to the cells of the IEEE 802.16e communication system. Each sub-channel of the C sub-channel sets An is orthogonal to the other sub-channels in the same sub-channel set while representing the uniform collision characteristics with respect to the sub-channels of different sub-channel sets.
If the C sub-channel sets An are allocated to each cell, the interference component from the adjacent cells can be averaged due to the uniform collision characteristics of the sub-carriers. So, if the amount of resources used in the adjacent cells is not changed, the validity of channel state information measured in a predetermined time unit can be maintained. In this manner, the IEEE 802.16e communication system can effectively manage the sub-channel based on the frequency reuse factor of 1. And although the inter-cell interference can be averaged, the SINR may be reduced from the interference components of adjacent cells. In particular, the SINR of the cell boundary region is significantly reduced.
Error correction coding with very low rate and modulation schemes with lower modulation order can be applied to the MSS located in the cell boundary region to ensure service coverage of the wireless cellular communication system. However, such error correction coding may degrade bandwidth efficiency, thereby significantly lowering the transmission rate for the MSS in the cell boundary region.
The IEEE 802.16e communication system with the frequency reuse factor K uses K unique frequency bands. Alternatively, the system logically divides the sub-carriers included in one frequency band into K sub-carrier groups. In accordance with an embodiment of the present invention, the sub-carriers included in one frequency band are divided into K sub-carrier groups and the frequency reuse factor K is managed based on the K sub-carrier groups.
Hereinafter, a procedure of creating the sub-channel in the IEEE 802.16e communication system based on the frequency reuse factor K will be described with reference to FIGS. 5A and 5B.
FIG. 5A is a schematic view illustrating the procedure of creating the sub-carrier in the IEEE 802.16e communication system based on the frequency reuse factor of K.
Referring to FIG. 5A, the sub-carriers formed in one frequency band are divided into K sub-carrier groups and the frequency reuse factor K is managed based on the K sub-carrier groups. In FIG. 5A, the frequency reuse factor is 3 (K=3). S sub-channels forming a predetermined sub-channel set An are divided into three exclusive sub-channel groups defined as Anα, Anβ, and Anγ.
FIG. 5B is a schematic view illustrating a group of sub-carriers corresponding to FIG. 5A allocated to sectors forming the cell of the IEEE 802.16e communication system.
Referring to FIG. 5B, under the frequency reuse factor of 3, three sub-channel groups Anα, Anβ, and Anγ are allocated to equal sectors of each cell. In an ideal case, inter-cell/sector interference rarely occurs so that the average transmission rate of the MSS located in the boundary region of the cell or sector may increase. However, the resources allocated to each cell or sector is reduced to ⅓, so the capacity of the cell or sector is reduced.
Hereinafter, a method of employing the frequency reuse factors 1 and K for improving bandwidth efficiency and system capacity of the IEEE 802.16e communication system will be described.
As described above with reference to FIG. 2, if the MSSs are located adjacent to the BS, relatively weak interference is applied to the MSSs in the cell center region. The MSSs in the cell center region may operate based on the frequency reuse factor of 1. In contrast, the MSSs located in the cell boundary region may operate with K>1 to reduce the interference applied to the MSSs from the adjacent cell or sector. That is, when simultaneously employing the frequency reuse factors 1 and K in the same cell, the interference in the boundary region of the cell/sector can be reduced by employing the frequency reuse factor 1 and the system capacity of the BS can be improved by employing the frequency reuse factor K.
However, if the IEEE 802.16e communication system employs the frequency reuse factors 1 and K without physically discriminating them, a relatively large interference component results. As a result, the SINR of the MSS having the frequency reuse factor K may be reduced and performance thereof will be significantly degraded. To solve the above problem, orthogonality is ensured between frequency resources having mutually different frequency reuse factors.
Hereinafter, a procedure of allocating frequency resources based on multiple frequency reuse factors in the IEEE 802.16e communication system will be described with reference to FIG. 6.
FIG. 6 is a schematic view illustrating the procedure of allocating the frequency resources based on multiple frequency reuse factors in the IEEE 802.16e communication system.
Referring to FIG. 6, if the IEEE 802.16e communication system uses N sub-carriers, the N sub-carriers are divided into G groups. Herein, each of the G groups consists of S sub-carriers, so that the following equation is satisfied:N=S×G
In addition, each of the G groups is divided into two sub-groups. The sub-groups include S1 sub-carriers and Sk sub-carriers, respectively.
First, a first sub-channel is created by selecting one sub-carrier from each of the G sub-groups. A second sub-channel is created by selecting one sub-carrier from each of the G sub-groups except for the sub-carrier that is already allocated to the first sub-channel. The above procedure may be repeated until all sub-carriers of the G sub-groups are allocated to the sub-channels. As a result, a set of S1 sub-channels is created. In addition, as mentioned above, it is also possible to create a new set An of C sub-channels having sub-carriers different from the above sub-carriers by varying the sub-carrier selection scheme. The sub-channels of the new set An are orthogonal to each other in the same sub-channel set while representing the uniform collision characteristics with respect to sub-channels in the other sub-channel set. The sub-channel set An is allocated to each cell/sector so that the cell/sector can be managed with a frequency reuse factor of 1.
Next, a first sub-channel is created by selecting one sub-carrier from each of the G sub-groups including Sk sub-carriers. A third sub-channel is created by selecting one sub-carrier from each of the G sub-groups except for the sub-carrier already allocated to the first sub-channel. The above procedure may be repeated until all sub-carriers of the G sub-groups are allocated to the sub-channels. As a result, a set of Sk sub-channels is created. The sub-channels are divided into K exclusive sub-channel groups and allocated to each of K cells/sectors, so that the cells/sectors can be managed with a frequency reuse factor K. In particular, since the sub-channels employing the frequency reuse factor 1 and sub-channels employing the frequency factor K include mutually different sub-carriers, interference may be prevented even if the frequency reuse factors of 1 and K are simultaneously employed.
However, there is no apparatus or method for controlling transmission power in the IEEE 802.16e communication system employing multiple frequency reuse factors.